As printing technology advances, paper manufacturers are faced with the increasingly rigorous demands of their customers for high quality paper that is economically attractive. For example, there is a keen demand for papers that meet high quality standards with respect to brightness, opacity, and dry and/or wet strength, and that, upon printing with any of a wide range of colorants, provide a water-resistant printed image. The customer further demands that such papers be amenable to use with a variety of printing techniques, including not only conventional printing techniques, but also "impact free" printing techniques, e.g., inkjet printing (particularly colored inkjet printing), laser printing, photocopying, etc.
In response, paper manufacturers have attempted to meet their customers' demands for such high quality paper through a process termed "sizing." "Sizing," which encompasses both "internal sizing" and "external sizing," affects the manner in which colorants and particularly ink interact with the fibers of the paper. "Internal sizing" involves introduction of sizing compositions within the entire fibrous mass at the pulp stage of paper manufacturing (i e., to the wet pulp, or more specifically, to the prepared papermaking furnish) before the stock is formed into a sheet, resulting in the distribution of the sizing composition within the entire fibrous mass that is subsequently used to produce the flat fibrous paper sheet. "External sizing" (also termed surface application, pasting, saturating or coating) involves application of a sizing composition to at least one surface of a flat fibrous paper sheet, so that the sizing composition is present on or in at least one of the two faces of the flat fibrous sheet. Paper and paper-based products are internally and/or externally sized to increase the resulting paper's strength, resistance to picking and scuffing, and resistance to undue penetration of water, organic solvents, oils, inks, and various types of aqueous solutions. Sizing is also used to improve the paper's smoothness and optical characteristics. In general, sizing is recognized to confer a number of advantages, including but not limited to the foregoing, with perhaps resistance to penetration of water and aqueous inks (e.g., bleed resistance) of utmost importance.
Various materials have been used as external and/or internal sizing agents, such as conventional and modified starches, polyvinyl alcohol, cellulosic derivatives, gelatin, rosin, proteins such as casein, natural gums and synthetic polymers. Internal sizing agents are generally referred to as acid, neutral, or alkaline internal sizes. Acid sizes are typically rosin based and precipitated by alum. Neutral sizes may also be rosin-based, and are used at near-neutral pH, while alkaline sizes are synthetic materials such as alkenyl succinic anhydride (ASA) and alkyl ketene dimer (AKD). Although these materials are effective to various degrees under certain conditions, use of each is associated with certain limitations. For example, it is often necessary to use large amounts of these conventional sizing agents in order to provide paper having the desired properties. However, the opacity and brightness of the paper substrate decreases in direct proportion to the amount of sizing agent applied to the paper. Moreover, as the amount of sizing agent and/or the cost of the sizing agent increases, the cost of producing the paper increases, making high quality papers having the desired characteristics prohibitively expensive and not economically viable. Certain sizing agents impart relatively poor bleed resistance and water resistance of imprinted inks, and thus must be used with insolubilizing agents to ensure production of a paper having satisfactory water resistance.
Conventional and modified starches are the most common sizing agents in use in the industry. Exemplary starch-based sizing agents include hydrophobic starches (see, e.g., U.S. Pat. No. 2,661,349), blends of hydrophobic and non-hydrophobic starches (see, e.g., U.S. Pat. No. 4,239,592; EP 350,668), and blends of treated starches and/or cationic starches (see, e.g., U.S. Pat. No. 4,872,951; EP 620,315; U.S. Pat. No. 5,647,898). However, while starches may provide improved porosity, these compounds generally do not provide for improved bleed resistance or water resistance of inks printed on the treated paper substrates.
Several synthetic sizing agents are presently available, and may be used in internal sizing and/or external sizing processes. Exemplary synthetic sizing agents include hydrophobic cellulose reactive sizing agents (see, e.g., U.S. Pat. Nos. 4,478,682; 3,840,486), cationic polymers (see, e.g., U.S. Pat. No. 3,006,806), and water-soluble, poly(aminoamide) resins (see, e.g., U.S. Pat. No. 4,478,682). Synthetic compounds have also been used to enhance the dry and/or wet strength of paper (see, e.g., U.S. Pat. Nos. 5,138,669; 3,058,873; 5,510,004; 5,659,011), either at the internal sizing or external sizing steps. However, synthetic sizing agents tend to be expensive, due to both the cost of the starting material and the amount that is required to provide a paper substrate having the desired characteristics.
Unfortunately, the use of sizing agents normally results in a decrease in the porosity of the final paper substrate. Thus, while the final sized paper substrate may have the desired brightness and opacity, it may not provide for a printed image that is of a desired optical density or color intensity. In addition, as the porosity of the paper increases, the paper becomes less amenable to various handling processes during manufacturing. For example, envelope manufacturers demand that the paper available to them have a relatively low porosity. If the porosity of the paper is too high, the paper is too stiff for handling by the automatic industrial devices for folding and sorting (e.g, devices of the "suction extractor" type) during envelope production. Moreover, in contrast to lower porosity papers, high porosity papers require slower machine speeds and refining and draining operations that have relatively high energy costs, both of which add up to decreases in plant productivity, efficiency, and cost effectiveness.
There is a need in the field for sizing agents and methods that provide an effective, cost efficient means for sizing paper, are amenable for use with a wide variety of paper manufacture and post-manufacture handling processes, and provide high quality, water-resistant images.